The present invention relates to a homogenous charge compression ignition engine (HCCI engine). In the HCCI engine of the present invention, the combustion mode can be switched between homogenous charge compression ignition combustion (HCCI combustion) and spark ignition combustion (SI combustion).
In recent years, homogenous charge compression ignition (HCCI) engines have been drawing attention, and various researches have been conducted on such engines. Excellent fuel economy and thermal efficiency and low emission can be gained from the HCCI engines. In some kinds of HCCI engines, fuel is directly injected into a combustion chamber during an intake stroke. That is, only air is drawn into the combustion chamber from an intake passage and fuel is mixed with air for the first time in the combustion chamber. However, in most of the HCCI engines, the fuel is mixed with air on the intake passage so as to generate air-fuel mixture. The air-fuel mixture is supplied to the combustion chamber from the intake passage.
The temperature increases and the pressure rises in the air-fuel mixture contained inside the combustion chamber as a piston rises at the time of the compression stroke of the engine, so that the mixture spontaneously ignites. One obstacle that needs to be overcome in terms of putting HCCI engines into practice is that the engine operating range, which allows homogenous charge compression ignition (HCCI) combustion to be stably controlled, is still narrow. In order to overcome the obstacle, there is a trend to implement HCCI combustion in stationary engines where the ordinarily used operating range is relatively narrow, for example, gas engines for GHPs (gas heat pumps).
In an actual operation of the HCCI engines, an engine low rotation speed range, a middle rotation speed range, a low load range and a middle load range are frequently used. It has been proposed that the combustion mode is switched between the HCCI combustion range and the spark ignition (SI) combustion range according to the running state of the engine. The spark ignition (SI) combustion is carried out in the high rotation speed range, the extremely low load range, and the high load range.
For example, Japanese Laid-Open Patent Publication No. 2003-106184 (JP 2003-106184 A) discloses a configuration in which a throttle is closed so as to return the air-fuel ratio to be stoichiometric when the combustion mode is switched from the HCCI combustion to the spark ignition combustion. Then, the lift amount of an exhaust valve is increased so as to reduce the internal EGR amount. That is, when the combustion mode is switched from the HCCI combustion to the spark ignition combustion, the throttle is first closed. This may cause insufficient intake air or low torque and makes the combustion difficult. Therefore, this may cause fluctuation of engine torque or occurrence of abrupt changes in the torque.
Japanese Laid-Open Patent Publication No. 2004-150383 (JP 2004-150383 A) discloses a configuration in which the combustion mode is first switched from the HCCI combustion to the stratified spark ignition combustion and then to the spark ignition combustion. Therefore, an in-cylinder injector is essential in the combustion chamber. When the combustion mode is switched from the HCCI combustion to the spark ignition combustion, the fuel injection amount is gradually increased in a state of the stratified spark ignition combustion. When the air-fuel ratio of the air-fuel mixture is reduced to a predetermined value, the fuel injection timing is advanced and the throttle opening degree is reduced. Accordingly, the combustion mode is switched to the normal spark ignition combustion in a stoichiometric state. That is, after the throttle opening degree is increased, the combustion mode is switched from the stratified spark ignition combustion to the normal spark ignition combustion in a stoichiometric state. If the throttle opening degree is reduced in this way, abrupt changes may occur in the torque.
An objective of the present invention is to suppress the occurrence of abrupt changes in the torque of the HCCI engine when the combustion mode is switched from the HCCI combustion to the spark ignition combustion.
One aspect of the present invention provides a homogeneous charge compression ignition (HCCI) engine that allows a combustion mode to be switched between an HCCI combustion and a spark ignition combustion. The HCCI engine comprises a combustion chamber, an intake valve and an exhaust valve. An intake variable valve mechanism changes an intake lift amount, which is a lift amount of the intake valve. The intake lift amount is set to a first intake lift amount in the HCCI combustion and the intake lift amount is set to a second intake lift amount in the spark ignition combustion. An exhaust variable valve mechanism changes an exhaust lift amount, which is a lift amount of the exhaust valve. The exhaust lift amount is set to a first exhaust lift amount in the HCCI combustion and the exhaust lift amount is set to a second exhaust lift amount in the spark ignition combustion. A controller controls the intake variable valve mechanism and the exhaust variable valve mechanism. The controller sets an internal EGR amount, which includes the amount of burned gas that remains in the combustion chamber at the time of the HCCI combustion and burned gas re-drawn into the combustion chamber through an exhaust port. The controller sets a negative valve overlap period during which both of the intake valve and the exhaust valve are closed such that not all the burned gas is discharged from the combustion chamber. When switching the combustion mode from the HCCI combustion to the spark ignition combustion, the controller executes following operations of a) and b) before switching the intake lift amount from the first intake lift amount to the second intake lift amount:
a): switching the exhaust lift amount from the first exhaust lift amount to the second exhaust lift amount while ensuring the internal EGR amount and
b): delaying a closing timing of the exhaust valve so as to reduce the internal EGR amount after switching the exhaust lift amount from the first exhaust lift amount to the second exhaust lift amount.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.